The methods for preserving genetic resources in fish are outlined in Table 1, Section 2. In this section those methods that are appropriate for aquaculture and stock enhancement are discussed in more detail.
Artificial selection of broodstock is broadly defined as the conscious selection of an individual or genetic group for the purpose of providing progeny for culture or stock enhancement. This selection can be based upon the performance of an individual (individual selection), of its progeny (progeny selection), of its ancestors (pedigree selection), of its contemporary relatives or on the potential of a useful combination with a parent of another genotype (hybridization selection). In all cases selection is practised in order to improve the performance of the progeny group above that of a progeny group from unselected parents.
There are many selection methods available by both the professional and non-professional aquabreeder (Moav, 1979). The proper application of the methods in a breeding programme can lead to useful and desired economic, aesthetic and ecological outcomes while still maintaining the genetic variability in the population. However, improper application of artificial selection or its application without sufficient background knowledge can lead to a deterioration of the genetic base.
Useful outcomes, both experimental (Cherfas, 1969) and economic (Kirpichnikov, 1973; Moav and Wohlfarth, 1973) have been documented for artificial selection in aquaculture. In these particular cases, undesirable outcomes have either been economically inconsequential or predicted (as in the case of experimental systems). In many cases, however, there have been undesirable results. This happens when simple brood stock selection and management per se leads to artificial selection.
The use of controlled mass rearing hatchery techniques dramatically increases the survival of larvae, post larvae and fry above that realized in nature. In theory, higher survival rates should increase the genetic variability available for natural and artificial selection, because they can “open up” the genotype of a genetic group in so far as new genetic combinations are allowed to survive. In the long term, however, culture conditions impose their own “natural” selection on the genotype (Doyle and Hunte, 1980; McCauley, 1978). This may or may not lead to a desirable outcome and must be monitored carefully.
Propagation of fish stocks in hatcheries for the establishment of a selected strain takes anywhere from ten to 50 generations. Fish breeders, therefore, might be tempted to maintain a minimum number of individuals. It is thus essential that they carefully weigh the costs of decreasing fitness and genetic variability against the economic advantages of maintaining such small numbers of individuals. As discussed in Section 4, fitness is expected to decrease so long as the effective population size of a brood stock is less than 50, and genetic variation in quantitative traits is expected to leak away unless the size is on the order of 500. If breeders choose to ignore these guidelines, they should do so in the knowledge that the genetic health to utility of their lines could be endangered.
Methods of artificial propagation, including the use of hormones, in vitro fertilization and development, are in wide use in aquaculture. For some species artificial propagation is necessary for life cycle control. In others, such propagation techniques are more efficient than natural breeding methods. The ways in which artificial propagation can affect the maintenance and study of genetic variation include:
1. Increase of genetic variability and genetic understanding. Numerous broods from different breeding pairs and from single maternal and multiple paternal (and visa versa) crosses allow not only the avoidance of inbreeding and bottleneck effects but the integration of selection operations with proper experimental designs. As pointed out in the report of the FAO Ad hoc Working Party on Genetic Resources of Fish (FAO, 1972) this integration allows both an improvement of stocks and a contribution to the knowledge of the genetic control of the production characters.
2. Aids in the conservation of stocks, strains, species and other genetic groups. Artificial propagation and life cycle control allows the conservation aqua-breeder to maintain valuable or potentially valuable stocks for current and future use.
3. Allows the development and use of specialized breeding methods. Gynogenetic procedures have been developed with the use of artificial propagation techniques. These procedures are used to create and maintain highly inbred lines which can be used to create and preserve useful homozygous genotypes. By maintaining numerous such homozygous lines in a population, overall genetic variability can be preserved despite the fact that each line represents severely reduced variability. Crosses of the lines can create heterozygous variability which may be economically useful because of hybrid vigour and/or because of the favourable non-heterotic combination of genotypes. Heterosis of 30–40 percent above that of the homozygous, gynogenetic lines can be expected in crosses of these lines.
4. Allows the efficient maintenance of effective population size. As discussed in Section 4 maintaining a minimum effective population size is essential for the conservation of genetic variability. Under natural conditions effective population size (Section 3,4) can vary widely. Through the use of artificial propagation methods man can control the effective population size most simply through the control of the numbers of breeding males and females.
5. Aids in interspecific hybridization. Normally, behavioural and anatomical differences preclude interspecific or intrageneric crosses between species which otherwise are reproductively compatible at least for the creation of F1 progeny. The use of artificial propagation techniques, especially those involving in vitro manipulation of gametes and in vitro culture of developing embryos in species which have maternal brooding, would greatly aid in developing interspecific hybrids.
Hybridization (or cross breeding) can be divided into two categories: (1) intraspecific: crosses between strains, stocks, land races, geographic populations within a species; (2) interspecific: crosses between species. Hybridization is practised to achieve either of two favourable outcomes. These are: (1) heterosis or hybrid vigour, which is defined in a broad sense as an increased performance or value (to the aquabreeder) of progeny above the average of the parental performances or value, and (2) non-heterotic effects which is the performance of the progeny as the result of simple combination of parental genotypes.
Specific hybridization is useful in producing a wide variety of new genetic combinations. Improvements in production above that attained for “land race” strains have been realized in extensive and intensive fish farming (Bakos, 1979; Wohlfarth and Moav, 1972; Yant, 1976). The expected heterotic effect can be increased using highly inbred parental lines (see Section 6.2). The desired outcome is the homozygosity of useful genes in a highly standardized and uniform product.
Interspecific hybridization is used not only to search for heterotic effects but also to search for favourable combinations of genotypes that control traits and performances that do not vary within a species to a large degree. For example, new kinds of social and feeding behaviour, better adaptation to environmental extremes in natural and controlled systems, and better adaptation to new husbandry systems can be realized by interspecific crosses. In addition, interspecific hybridization can be used to create desirable sterile or monosex progeny groups which also display favourable production performance. For example, a sterile triploid hybrid between the big head and grass carp has been developed (Marian and Krasznai, 1978). This sterile hybrid can be introduced to ecosystems for weed control with no danger of overpopulation or crossing with wild populations. The creation of monosex broods of Tilapia sp. brought about by interspecific hybridization is being intensely researched (Pruginin et al., 1975). As yet no one combination has consistently resulted in a monosex brood. However, crosses leading to highly skewed sex ratios are possible and are being utilized.
Interspecific hybridization is not rare in nature (Hickling, 1968; Hubbs, 1955; Schwartz, 1972; Slastenenko, 1957). However, to date most human attempts at interspecific hybridization in fishes have been directed at exploited Salmonid (Chevassus, 1979), Cyprinid (Bakos et al., 1978), and Centrarchid (Childers, 1971) fishes. However, in all fishes, unexploited species represent a vast repertoire of behavioural, physiological, nutritional and other adaptations which may prove useful in making larger impacts in genetic improvement than would be obtained through the use of intraspecific genetic manipulation. The development and use of artificial propagation and in vitro techniques will greatly aid in this endeavour.
There are numerous examples of activities based on the concept of augmentation of natural population production by introduction of hatchery raised, “cultured” juvenile fishes into natural systems. Varying levels of success have been achieved. In some cases, for example, the early attempts to augment cod (Gadus morrhua) production in the North Atlantic Ocean have not been obviously positive. In closed systems such as lakes, inland seas, and reservoirs, however, there have been obvious improvements in the productivity of various species.
It is apparent that natural fish stocks undergo genetic alteration through addition and deletion of genetic material. Although quite a lot of attention has been given to effects of competition between “wild” and hatchery raised fish, for example in salmonids, very little has been given to changes which may result to the genomes of the wild stock by mixture with artificially raised stocks. In some cases parental material is taken directly from the population into which the hatchery fish are subsequently released. In this case, the principle hazard would be the possible loss of genetic diversity through long-term inbreeding of the parental stock. Where there is relatively low rate of natural reproduction or where very few spawners are used to provide most of the recruitment, random loss of genetic variation (genetic drift) would increase. When, as is often the case, one hatchery is used to provide seed for stocking many lakes, the effects are much more likely to be important. Flick and Webster (1976) have reported higher survival of both wild and hybrid domestic strains of brook trout after release in small natural lakes compared to pure hatchery strains. In salmonids and carps, quite a number of distinct hatchery strains have been developed.
Genetic differences have been demonstrated between fish populations in different streams of the same river basin, between nearby lakes, and between sub-populations within the same body of water (Section 3.4). There is, however, little direct evidence concerning the degree to which such variation among stocks affects “fitness” in the different habitats. Recent biochemical and physiological studies have shown, however, the existence of correlations between gene frequencies and environmental variables, such as temperature (Place and Powers, 1979; Powers, et al., 1979, Koehn, 1969; Merritt, 1972). Thus, local ecotypic adaptations of fish populations are to be expected and the genetic and ecological consequences of intraspecific transfers may be undesirable.
In order to maintain high levels of genetic variability in cultured fish, it would be necessary to maintain large numbers (i.e., hundreds) of breeders. This is expensive and often not feasible. The introduction of the cryopreservation of gamets which can be used when necessary, thereby reducing the genetic erosion that invariably results from inbreeding, allows one to freeze the sperm of numerous male donors from all possible strains.
Essentially the method involves stripping sperm, from males, diluting the sperm with appropriate extenders and life protectors, then freezing in liquid nitrogen or on dry ice followed by liquid nitrogen. The exact methodology is somewhat different for each species (e.g., Horton and Ott, 1976; Rosenthal et al., 1978. Once the method has been refined for a given species, sperm from thousands of donor males can be kept for years thereby preserving the genetic resources for (1) cultured fish stocks, (2) unique natural populations, and/or (3) species that are threatened. At present methods for freezing fish eggs or embryos are not technically feasible but research in that area is being actively pursued (e.g., Whittingham and Rosenthal, 1978).
